Move prost codegen to outdir #1046

Job	Run time
build-vegafusion-python-linux-arm64	15m 38s
build-vegafusion-python-osx-arm64	20m 1s
build-vegafusion-python-osx-64	38m 22s
build-vegafusion-python-linux-64	15m 0s
build-vegafusion-python-win-64	39m 3s
build-vegafusion-server-linux-64	14m 25s
build-vegafusion-server-osx-64	37m 30s
build-vegafusion-server-linux-arm64	15m 5s
build-vegafusion-server-osx-arm64	19m 57s
check-rust	2m 29s
build-vegafusion-server-win-64	41m 23s
build-vegafusion-wasm	9m 20s
test-rust (ubuntu-20.04)	21m 36s
check-vegafusion-python	10m 46s
test-rust (windows-2022)	45m 19s
test-rust (macos-14)	27m 16s
test-vegafusion-python-linux-64	25m 41s
test-vegafusion-python-osx-arm64	51s
test-vegafusion-python-win-64	1m 55s
bundle-vegafusion-server-builds	6s
bundle-vegafusion-python-wheels	7s
	6h 41m 50s

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